The invention relates to a method for operating an occupant safety device as well as a control unit, in particular for performing the method.
A method of this kind is known from DE 37 29 785 C1 (or its corresponding U.S. Pat. Reissue No. 34,637). As described in DE 37 29 785 C1, the supplied electrical power is to be established by measuring the voltage at the storage element, comparing it with a desired value and interrupting the power supply if it falls short of this value. This method can be expanded in accordance with column 5, lines 34 to 42 of DE 37 29 785 C1 by measuring the current flowing through the triggering means at the same time as measuring the voltage. The description of DE 37 29 785 C1 is limited to these two examples of embodiment in which in both cases the voltage is given as the variable to be measured. The voltage of the storage element, however, displays a major disadvantage when determining the supplied energy because it depends on the charged state of the storage element. In DE-OS 37 38 862 from the same applicant as DE 37 29 785 C1, this disadvantage is indeed recognized in principle (cf. column 1, lines 29 to 40 of DE-OS 37 38 862), but only a time control is considered as solution. As described in DE 37 29 785 C1, it is not possible to supply more than one triggering means, for instance airbag squibs, in the same time from one common storage element with an controlled amount of energy.
Another disadvantage of DE 37 29 785 C1 is that the accuracy in determining the power supplied to the triggering means by measuring the voltage depends very much on the conductor resistance. In practice, it has been found that the total resistance comprising the triggering means as such (2 ohm) and the conductor (varying between 0 and 4 ohm) depends heavily on the latter. The power that is effectively supplied to the triggering means can therefore be considerably less than that which can be determined from the voltage at the storage element.
From DE 196 24 357 C1, an ignition circuit output stage is known for triggering occupant safety systems in motor vehicles, where there is mention not only of a deactivated but also of an activated and limited power output in which the ignition power supplied to the ignition device is limited to a predetermined level. This is effected, however, by means of a switched-mode power supply in which the ignition current is held between a first and a second value of the current by opening and again closing the switching means accordingly. In the ignition circuit, a coil is needed for intermediate energy storage in order to be able to feed the supplied ignition power to the ignition device even when the switching means are open. This ignition circuit output stage thus makes possible the intended limitation of the supplied amount of power only by means of current limiting and requires not only numerous additional components, in particular a coil and a Schmitt trigger for the switched-mode power supply, but also an additional time limiting facility because otherwise an unlimited amount of power can be supplied over a period of time in spite of the limited current.
The object of the invention is to specify a method for operating a safety device that is accurate and independent of the conductor resistance. Furthermore, a suitable control unit will be presented.
A variable that is proportional to the current flowing through the triggering means has the decisive advantage of being equal everywhere in the ignition circuit and of being independent of the conductor resistance. Such a variable that is proportional to the current can be obtained very easily from an electrical circuit and, by squaring and integrating using both circuitry and software means, a variable can be determined from it that is proportional to the supplied power or energy and that is compared with a power or energy limiting value which, on being reached, results in the power being switched off. It is also possible supplying more than one triggering mean at the same time from one storage element for power supply by measuring the current through each triggering mean individually.
By programming the power or energy limiting value rather than predefining it from the start, a control unit can be adapted individually either immediately before installing it in the vehicle or even during the product life or depending on the present operating condition of the vehicle.
Furthermore, not only an activated or regulated mode but also a deactivated or unregulated mode can be provided which means that the control units can be used not only for vehicles with an activated (regulated) power supply but also for vehicles without it and additionally it is possible to deactivate the power limiting control under certain operating conditions.